I've seen this situation before, the 3Com boot floppy solved the problem.
Are these old nics?
Can your RIS clients get an address if they are booted with the F12 key
rather than the boot floppy?
I've only used the PXE boot option, not TCP/IP, they did give an e-mail
address for support: (e-mail address removed)
Since the 3Com PXE failed and MS's PXE boot floppy failed, I'm wondering if
your routers/switches are timing out. Do you have spanning tree enabled?
You may need to disable it temporarily to test and perhaps lengthen the
timeout.
Do your routers have a helper address for both the RIS server and the DHCP
server?
Watch the clients just before they fail, the proper sequence is: BootP,
DHCP, BINL, TFTP. The BINL can appear really fast, so if that doesn't
appear then no biggie, but it is important to see where the failure occurs.